In an inhomogeneous assay, the concentration of a targeted bio-molecule can be determined by measuring the surface concentration of the targeted bio-molecule or beads [that are representative for the targeted bio molecule] bound at the sensor surface. As an example, one can think of a competitive assay where the binding surface (substrate) is covered with target molecules. The beads may be covered with specific [for the target molecule] antibodies and are dispersed in a fluid that contains the target molecules. The free target molecule in the sample competes with the immobilized target molecule on the sensor surface for binding to the antibody-coated bead. In case of a low concentration, the chance that an antibody binds with a target molecule at the sensor surface is higher than the chance that an antibody binds with a target molecule in the solution. By measuring the surface concentration of beads that are bound at the substrate, one can determine the concentration of the target molecule. Accurate measurement of the concentration however requires a highly surface specific detection scheme that is sufficiently insensitive for beads in the solution. A prior art sensor utilizes an evanescent field mode that is generated by illuminating sub diffraction limited apertures on a sensor surface, in particular, the so called wire grid, which has aperture defining structure defining a largest in plane aperture dimension larger than the diffraction limit and a smallest in plane aperture dimension smaller than the diffraction limit. Typically, the detection volume of the beads is provided between said aperture defining structures—so that the beads through electromagnetical interaction with the evanescent field mode, provide a detectable radiation—for instance, luminescence radiation or a change of reflectivity/transmissivity of the sensor due to the presence of beads. Generally, the sensitivity of a wire grid biosensor depends on the fraction of the input/excitation power that is coupled into the evanescent detection volume. In illumination modes wherein the illumination has an angle with respect to the surface normal, the power that is coupled into the detection volume will be reduced. A desire exists to increase the excitation efficiency, without needing to utilize higher intensities for the detection of target components.